Related: All bootloaders should be unlockable if the device owner opts to do that. Manufacturers/carriers should not be involved in that process at all.
You can, just not if it's locked to a specific carrier.
Carriers often do that to prevent people from rooting their phone or removing Carrier identification to resell the phone.
Stupid design either way
It may not be the case anymore but in the past you could go into Verizon and ask/tell them to unlock the phone once it was paid off and they would. I did it probably 12 years ago at this point but also haven't had Verizon in 8+ years